A pandemic could begin with one infected person flying across the world, shedding pathogens along the way. It's a terrifying scenario, but it probably wouldn't have happened thousands of years ago. Not because we didn't have airplanes – but because humans had not yet become a crowd species.

Today in the Guardian, Alok Jha reports on the latest research into global pandemics. He focuses especially on zoonotic pandemic diseases, which jump from one species to another. It's likely that our next pandemic will be zoonotic, just as many forms of influenza have been in the past.

For such a disease to take hold, humans must be living in close proximity to animals. Equally important, humans must live in close proximity to each other, allowing the pathogen to spread quickly.

Reports Jha:

Viruses and other pathogens continually flow between species, often with no effects, sometimes mutating, once in a while causing illness. This mixing is known as "viral chatter" and the more different species come into regular close contact, the higher the chances of a spillover event occurring.

"This is how viruses have always worked, the big change is us," says Mark Woolhouse, a professor of infectious disease epidemiology at the University of Edinburgh. "The big change happened probably several thousands of years ago when we became a crowd species and that gave these viruses new opportunities which they hadn't had before in humans. Ever since then, from time to time a new virus has come along to take advantage of this new, very densely populated, crowded species – humans – that it can now spread between much more easily. That process is still happening; the viruses are still discovering us. We like to think we discover viruses, but it's also the viruses discovering us."

In a sense, epidemics and pandemics are city problems. They arose thousands of years ago, when humans first began to build city-like settlements, where they lived year-round in shelters ringed by farmland. Once humans were living in such close proximity, we became the perfect hosts for viruses. Though city life has brought us many benefits, it has also left us vulnerable to near-extinction.

Read more about how pandemics happen, and how we could stop them, at the Guardian.